In recent years, rapid increase of the Internet traffic has pushed the amount of data transmission sharply, and it has been demanded to increase the operation speed, as well as decrease the size and the cost for optical transmission and receiving equipment or optical components. Under the background described above, development has been conducted vigorously for silicon photonics that realize optical components by using silicon materials for making photonic integrated circuits (PIC) through CMOS processes instead of using optical components made on compound semiconductors such as GaAs and InP substrates directly.
For passive optical components such as optical waveguides, wave guiding by the use of the silicon material has already been confirmed. FIG. 1a shows a conventional semiconductor laser apparatus 100 including active gain medium material stack (LD stack) 102 coupled to passive semiconductor material stack, such as for example the silicon layer of a silicon-on-insulator (SOI) stack 103, which is disclosed in a Ph. D. thesis of Stevan Stanković from Ghent University, ISBN 978-90-8578-594-1, NUR 959, Copyright: D/2013/10.500/27. As shown, the semiconductor laser apparatus 100 includes an optical waveguide 104a disposed in the single layer of semiconductor material 104 which is formed on a silicon substrate 106. The optical waveguide 104a includes an optical cavity 111 defined along the optical waveguide 104 between two reflectors (not shown). In Si photonics based optical transmitters, a III-V Laser Diode (LD) stack 102 is hybridly integrated/bonded with a single silicon waveguide core 104a on the SOI substrate 103 to realize single mode (SM) optical confinement using a thin layer of low dielectric constant polymer such as BCB or DVS BCB 108. FIG. 1b shows a perspective view of the single silicon waveguide 104 in FIG. 1a. Specifically, the silicon waveguide 104 includes a single rib 104a acted as a core in the middle portion of the waveguide 104.
U.S. Pat. No. 7,016,587 B2 also discloses such a single silicon waveguide core, as shown in FIG. 1c, the silicon rib waveguide 130 includes a silicon substrate 125, a single crystal silicon layer 126 and an insulator bonding layer 127 therebetween. A silicon rib 134 between two parallel trenches 135, 136 is formed on the single crystal silicon layer 126, by any suitable patterning process. As shown, a silicon nitride layer 138 is deposited on the patterned surface of the single crystal silicon layer 126, including the trenches 135, 136 and silicon rib (or core) 134 therebetween.
However, the waveguide with a single silicon core (or rib) 104 or 130 will generate high light propagation loss, which requires high index difference between the LD stack and the waveguide of the SOI stack. Also, due to lack of high thermal conductivity material just in between the full length of LD stack 102 and the single silicon core 104a or 134, the heat dissipation is inefficient, and the junction temperature (Tj) of the LD stack 102 increases and degrades the life time of device accordingly.
FIG. 1d shows the theoretical observation that indicates the coupling of light into the single silicon waveguide core 104a. By this token, the light transmission loss in the waveguide core 104a is higher, due to some of optical confinement of light is presented in LD stack of single silicon waveguide core 104.
So for high speed, low cost and highly reliable Si-photonic transmitters, it's desirable to provide a lower loss waveguide and a method to make lower loss waveguide that simultaneously offers SM optical confinement and improved heat dissipation from active region that reduces the junction temperature of LD with high mechanical bond strength.